(163f) Distributed Product Quality by the Example of Moisture Content of Granular Materials in a Continuous Fluidized Bed Dryer

Solids processing of particulate materials, such as drying, are of major interest in the chemical industry. Various parameters ? for material behavior, apparatus design and operating conditions ? have influence on the resulting product quality. Such disperse systems can be good modeled by means of population balances; these describe the temporal change of number density distributions of individuals of the disperse phase with respect to different external and internal coordinates.

An example for applying population balances is continuous fluidized bed drying. The traditional approach towards modeling of this operation considers the disperse solids as one phase with average properties like particle size, moisture content and enthalpy [1]. But it was found, by comparison with experimental data, that such a model tends to underestimate the outlet moisture content [2]. Model accuracy can be significantly improved by use of population dynamics. Due to different retention times of particles, a moisture distribution in the product stream is expected. This can be modeled by introducing an additional property coordinate, the age of particles.

The first simulation results of the model, which considered this property coordinate in the balances of the number of density, the enthalpy and the mass of water, are presented in this work. It is shown that this model yields considerably better values for product moisture at the dryer outlet. The measurement methods to validate the simulation results and the associated problems are presented. Our first results (see Figure 1) verify that the outlet moisture of mono-sized particles is distributed over the relatively wide range, reflecting different retention times in the apparatus.

Based on these results, the set of product characteristics can be arbitrarily extended to other (outlet) properties, such as color or bioactivity, which depend on the history of the particle concerning temperature and water activity.